strength exceed capacities of available equipment, further evaluation is required. Options may include:
(1) Selecting a slower towing speed.
(2) Using additional or more powerful tugs.
(3) Decreasing resistance by changing the tow's characteristics, routing and/or schedule.
5-3 TOWLINE TENSION.
There are three components of towline tension
a. Tensions generated by the tow
(1) Steady tow resistance
(2) Slow dynamic cyclic loads caused by the tow's yawing, sheering and surging
(3) More rapid dynamic cyclic loads caused by relative seakeeping motions of tug and tow
b. Towline resistance or hydrodynamic drag (adds to the tug's propulsion requirements)
c. Vertical components of wire catenary (contributes to the total tension of the towline itself but not to tug
ESTIMATING STEADY TOW RESISTANCE.
The steady tow resistance (RT) may be estimated using the
RT = RH + Rp + Rw + RS
RH = Hydrodynamic hull resistance of the tow
Rp = Hydrodynamic resistance of the tow's
Rw = Wind resistance of the tow
RS = Additional tow resistance due to sea state
Appendix G provides a met hod for predicting each of the steady tow resistance components. Effort can be saved by
computing the resistance for two or three different speeds for later comparison to tug capabilities. Remember to consider
towing speed limitations cited in Paragraph 3-4.2. Likewise, wind and sea state resistances should be computed for best
and worst expectations as well as for the most probable conditions of each assumed tow speed.
RESISTANCE OF THE TOWLINE. The hydrodynamic resistance of the towline, R ire, can be significant for a
typical wire hawser tow rig. This resistance is dependent upon the size, length and catenary of the towline, which in turn
are dependent upon characteristics of the selected tug and towing speed.
If the tug has not been selected, add 10 percent of the tow resistance of the towline. Accordingly, to approximate the
total towline pull required of the tug, multiply by 1.1 the values of RT obtained using the methods described in Paragraph
If the tug already has been selected, or if tugs are being evaluated, Table 6-1 provides a more refined estimate for the
hydrodynamic resistance of the towline R ire. In this case:
R = RT + Rwlre
The added resistance of the towline when using a synthetic hawser can be ignored since it will be small compared to the
resistance of the tow.
CYCLIC AND OTHER LOADS IMPOSED ON THE TOWLINE.
Towline loads imposed by the seakeeping motions and the vertical component of the towline catenary